In a typical live music concert, multiple microphones (acoustic pick-up devices) are positioned close to each of the instruments and vocalists. The electrical signals from the microphones are mixed, amplified, and reproduced by loudspeakers so that the musicians can clearly foe heard by the audience in a large performance space.
A problem with conventional microphones is that they respond not only to the desired instrument or voice, but also to other nearby instruments and/or voices. If, for example, the sound of the drum kit bleeds into the microphone of the lead singer, the reproduced sound is adversely effected. This problem also occurs when musicians are in a studio recording their music.
Conventional microphones also respond to the monitor loudspeakers used by the musicians onstage, and to the house loudspeakers that distribute the amplified sound to the audience. As a result, gains must foe carefully monitored to avoid feedback, in which the music ample lying system breaks out in howling that spoils a performance. This is especially problematic in live amplified performances, since the amount of signal from, the loudspeaker picked up by the microphone can vary wildly, depending on how musicians move about on stage, or how they move the microphones as they perform. An amplification system that has been carefully adjusted to be free from feedback during rehearsal may suddenly break out in howling during the performance simply because a musician has moved on stage.
One type of acoustic pick-up device is an omni directional microphone. An omni directional microphone is rarely used for live music because it tends to be more prone to feedback. More typically, conventional microphones having a directional acceptance pattern (e.g., a cardioid microphone) are used to reject off axis sounds output from other instruments or voices, or from speakers, thus reducing the tendency for the system to howl. However, these microphones have insufficient rejection to fully solve the problem.
Directional microphones generally have a frequency response that varies with the distance from the source. This is typical of pressure gradient responding microphones. This effect is called the “proximity effect”, and it results in a bass boost when the microphone is close to the source and a loss of bass when the microphone is far from the source. Performers who like proximity effect often vary the distance between the microphone and the instrument (or voice) during a performance to create effects and to change the level of the amplified sound. This process is called “working the mike”.
While some performers like proximity effect, other performers prefer that over the range of angles and distances that the microphone accepts sounds, the frequency response of the improved sound reproducing system should remain as uniform as possible. For these performers the timbre of the instrument should not change as the musician moves closer to or further from the microphone.
Cell phones, regular phones and speaker phones can have performance problems when there is a lot of background noise. In this situation the clarity of the desired speakers voice is degraded or overwhelmed by this noise. It would be desirable for these phones to be able to discriminate between the desired speaker and the background noise. The phone would then provide a relative emphasis of the speaker's voice over the noise.